1. Field of the Invention
The present invention relates to a process for preparing a slip casting mold for casting a slip, i.e. a slurry containing, for example, refractory powder such as ceramic powder metal powder, or carbon powder to produce cast articles, or a metal casting mold for casting an iron alloy, a copper alloy, an aluminum alloy, or the like to produce metal products, and more particularly to a process for preparing a mold suitable for cast articles requiring a core and a master mold having such complicated shape that it will not allow the molding to be drawn out because of, for example, back draft.
2. Description of the Prior Art
In cast articles having a complicated shape of appearance and/or hollow, there has heretofore been generally employed a method wherein a mold formed by a combination of a number of master molds and cores as desired is used. However, this method involves many problems in that a number of steps are required for preparing and assembling master molds and cores, that burr formation frequently occurs, and that the dimensional accuracy is apt to be low.
A method of solving these problems is disclosed, for example, in British Pat. No. 1,482,436. In this method, a mold comprises a portion of a complicated shape constituted by an organic material and a portion of a simple shape constituted by gypsum which serves to absorb water contained in a slip to be cast into a mold to thereby solidify the slip. The organic material is then dissolved away with a solvent for producing a cast article in a wet state (green body).
However, this method is devoid of the following consideration.
(i) The residue of the organic material is liable to remain locally on the surface of the green body. The more complicated the shape of the green body, the more difficult the removal of the residue. PA0 (ii) In some cases, a long time is required to solidify the green body because the portion of the mold having a water absorptivity has only a small area. PA0 (i) Smoke and soot are generated during the course of high-temperature heating of the mold. Part of the soot remains on the inner wall of the mold. PA0 (ii) This method is applicable only to a mold prepared by using a binder and an aggregate having extremely good heat resistance.
There is another method of preparing a mold by using a foamed polystyrene pattern in place of a wax pattern used in the lost-wax process employed in precision casting of metal. However, this method, too, has several problems. An about 5 to 10 mm-thick refractory layer is stuck onto the surface of a foamed polystyrene pattern having substantially the same shape as that of a product. After solidification, th mold thus formed is contacted with a solvent such as trichlene for 1 to 3 hours to dissolve the pattern, a major part of which is dropped and removed outside the mold. However, the residue of polystyrene remains in a form of a layer substantially all over the inner wall of the mold. In this case, a particularly large amount of the residue remains in the hollow portion having a shape that will make it difficult to pour out the residue. For this reason, the mold is heated at high temperatures (about 1,000.degree. to 1,100.degree. C.) to burn the residue remaining in the mold. In this case also, there are the following problems.
U.S. Pat. No. 2,830,343 discloses a full mold casting method (filled mold casting method) comprising embedding a foamed polystyrene pattern in a molding sand, and directly pouring a molten metal into the pattern to let the pattern disappear by the heat of the molten metal and fill the space occupied by the pattern with the molten metal. Despite a small number of steps required in this method, however, there is liability of frequent occurrence of defects of such as impression formed in the skin of the mold by the residue of the burnt pattern and blow caused by a combustion gas of the pattern. In this case, the quality of the product is reduced.